1. Field of the Invention
The present invention relates to a scanning and imaging lens and an optical scanning device.
2. Description of the Related Art
A scanning and imaging lens for use in an optical scanning device is required to have a proper f.THETA. characteristic in order to ensure a constant velocity in optical scanning. Further, the curvature of field of such a scanning and imaging lens must be corrected in order to reduce a variation in the diameter of a beam spot. In order to meet these requirements, the surface of the scanning and imaging lens needs to have a unique configuration (see Japanese Laid-Open Patent Application no. 7-35996).
Hereinafter, the term "direction corresponding to the main-scanning direction" is used to denote a direction, in an optical path between a light source and a scan surface, that corresponds to the main-scanning direction. The term "direction corresponding to the sub-scanning direction" is used to denote a direction, in an optical path between a light source and a scan surface, that corresponds to the sub-scanning direction.
A polygon mirror is normally used as a beam deflector for deflecting a light beam incident on a scanning and imaging lens. The center of rotation of a deflecting and reflecting surface of a polygon mirror is not located in the deflecting and reflecting surface. A beam may be focused so that a line image extended in a direction corresponding to the main-scanning direction is formed in the vicinity of the deflecting and reflecting surface, in order to correct a so-called facet inclination. In such a construction, a sag may occur. A sag is a variation of a relative position at which the line image is formed with respect to the deflecting and reflecting surface, the variation being asymmetrical with respect to an optical axis of the scanning and imaging lens and occurring in response to the rotation of the polygon mirror.
The surface of the conventional scanning and imaging lens is symmetrical with respect to the optical axis in the direction corresponding to the main scanning direction. Therefore, when a sag occurs, the curvature of field of and the constant-velocity characteristic of the lens is significantly degraded. Such a degradation in the performance is conventionally corrected by a shift of the scanning and imaging lens or a tilt thereof. However, such means of correction is far from satisfactory.